In general, a spark plug includes a center electrode, an insulator, a metal shell, and a ground electrode. The metal shell of the spark plug has a tubular shape having an end surface and an inner peripheral surface. The end surface of the metal shell is connected to the ground electrode. A gap for preventing spark leak (lateral spark) is formed between the inner peripheral surface of the metal shell and the insulator holding the center electrode. Spark leak is a phenomenon in which spark discharge is generated at a portion different from a spark gap between the center electrode and the ground electrode.
JP-A-2003-223968 and JP-A-2011-175985 disclose techniques in which the end surface and the inner peripheral surface are formed to the metal shell and then the ground electrode is welded to the end surface of the metal shell. When the ground electrode is welded to the end surface of the metal shell, welding sag may protrude to the inside of the metal shell and such welding sag becomes a cause of the spark leak. Furthermore, JP-A-2003-223968 discloses a technique in which the ground electrode is welded to the end surface of the metal shell and then the welding sag protruding to the inside of the metal shell is removed by a shearing process or a cutting process.
In the techniques disclosed in JP-A-2003-223968 and JP-A-2011-175985, there is a problem that it is difficult to sufficiently further reduce a protrusion amount of the welding sag on the inside of the metal shell from the viewpoint of ensuring a bonding strength of bonding the ground electrode to the metal shell and preventing damage with respect to the inner peripheral surface of the metal shell. Particularly, when a size of the spark plug is reduced, since an influence of the welding sag to the spark leak becomes noticeable, it is necessary to further reduce the protrusion amount of the welding sag.